Lots of weather going on right now and big changes brewing over the next two weeks, so let's dive right in and take a look at what we see coming down the road.

Meteorologist are watching this last week of December with great interest right now, as there appears to be several legitimate chances at winter weather across the southeast, and for a while now the models have been hinting at this upcoming pattern change that is getting ready to take place beginning next week.

By now everyone knows that November was well below normal for us here in Georgia and much of the eastern US. Here are two maps, one shows that Georgia had the 4th coldest November ever, the other map shows how far below normal we actually were. I think everyone got spoiled with those early winter like temperatures and expected those to last through the rest of winter! No such luck though. However, the warm up we had during the first week of December was expected as well as the one coming up for next week.

November was the 4th coldest ever for Georgia

Georgia was 4.4 degrees below normal for November

The west coast is currently being flooding with moist Pacific air as a big upper level low spins just off the coast. This rainy pattern is all part of the El Nino like pattern that is beginning to take shape. What will begin to happen over the next several weeks is that a large ridge will begin to form just off the west coast which will help to shunt the storms to the south and into the path of the southern jet stream. With this high in place, systems will begin to traverse the southern portion of the US and follow tracks that are very conducive to winter time storms along the Gulf coast (we hope). The track along the coast is ideal for us here in north Georgia. If you look at this map of the track of the storm that brought us a White Christmas in 2010, you'll see what I'm talking about. That type of low track is what we want to be on the lookout for.

Track of 2010 Georgia White Christmas winter storm

So now that we know what we want to look for, let's see what we can find right now.

The two maps below are from the GFS ensembles. The map on the left shows 500 Mb height anomalies for December 25th. This is the upper air pattern that the GFS is depicting right now, and it shows a large trough over the eastern US with blocking near the west coast and Greenland. Blocking like this forces the cold air move south as it's not able to move very far east. The map to the right is the ensemble precipitation forecast. Each map within that map is a run from an individual ensemble member. You can see that many of the members are beginning to show some major precipitation falling over the southeast over that time period. At this point the Euro isn't quite on board as much as the GFS but it still has a similar look to it.

GEFS (ensembles) 500 MB height anomalies for December 25th. Notice the blocking that's beginning to take shape off of the west coast and near Greenland. Those "blocks" force the cold air south into the central part of the country. Maps from Dr. Ryan Maue @Weatherbell.com

This is the GEFS precipitation forecast. These are individual ensemble members forecast. Looks very moist in many of the members and leads us to believe a large storm is in the works. Maps from Dr. Ryan Maue @Weatherbell.com

Examples of -AO, +PNA, -EPO

We're watching the teleconnection patterns with great interest right now. What we want to see is a -AO and -NAO in order to get the blocking that we need to bring the cold air south, and the GFS ensembles are showing that both should be heading toward negative values by the end of the month. We're also looking for a +PNA and -EPO signal and both of those seem to be in our future. The GFS ensembles show the EPO dropping off the charts almost. The positive PNA sets up the ridge off the west coast, the -EPO retrogrades the stubborn low pressure over the Gulf of Alaska westward, and the -AO provides over the top blocking. The map shows a +NAO but the -NAO will build as the winter progresses and will help to lock in the cold over the US. Here's a great map that depicts that setup. Now all the need to get is cross-polar flow from Siberia and brrrrrrr.

Post from various meteorologist...

I want to share this post from WXRisk that also helps to explain what is about to occur over these last several weeks.

I have been talking about the December 22-26 change for over a week now so the TREND IS MY FRIEND.One of the reasons why I am not particularly bullish about the event/threat for December 21 is that the overall pattern on a hemispheric scale ...is not very favorable. Yes the models DO show what appears to be cold HIGH in a pretty good set up ...at least for the beginning of the event. The problem is that the flow at 500 MB across Canada is Pacific and ZONAL and has no arctic connection.

Thus the HIGH is NOT very cold and has to be the perfect ideal position when the coastal low begins in order to keep some of the precipitation snow.The only reason why this is potential threat to begin with is the development of the big ocean low over southeastern Canada (the 50/50 LOW).

The NAO is strongly positive which along with the Pacific pattern strongly interdicts the supply of cold air into the Northeast US.

But beyond the December 21 event... if we use the overall hemispheric pattern as a starting point we can truly see some significant changes taking place across the hemisphere by December 25th.

Here is the 324 hour EURO Ensemble Mean.... It shows several important things... many of which are self explanatory.

The 5 day CFS from from the website Tropicaltidbits.com web site has been showing this for while.... and increasingly amplified +PNA and -EPO pattern . Now the euro ensembles are showing this as well. See image.

The one thing that should be noted is that we do not on this map yet see a -NAO and the Polar Vortex is still very far to the north over Baffin island.

Here is the week 3 and 4 500 MB anomaly maps from the CPC. I have drawn in some key features for ease of comprehension. The CFS clearly indicates that the pattern amplification is going to really picked up speed in week 3 and week 4 . The -EPO goes absolutely ballistic in week 4 and strongly resembles what we saw last January

But again ...even though there IS a -AO we do not have -NAO yet and here is why.

Next week as the zonal flow across Canada begins to amplify and it will take on the WIDE and BROAD shape. We can all see that. Because of this, the Polar Vortex is forced to over to Baffin Island in far Northeast Canada. This in turn means any RIDGE is forced over Iceland or the UK or Norway ...and the NAO is either positive or neutral.

BUT as the western Canada ridge amplifies ... it will get pulled WEST because of the --EPO development. This will cause western Canada ridge to sharpen up.... which in turn will allow the PV to shift SW into Hudson Bay, and THAT that in turn will allow the NAO to go Negative in January.WXRisk.com

Comparison of the CFSv2 Forecast for December vs. December 1-10 Outcomes: If the ensemble guidance is representative, parts of the eastern U.S. and eastern Canada (eastern Ontario, Quebec, New Brunswick) could finish December with cold anomalies, especially with the increasingly likely pattern change in the medium-term.Even as it is near the end of its fantasy range, the 12z run of the operational GFS brings a shot of Arctic air into the CONUS. This outcome is supported by some of the earlier ensemble guidance and Day 21-25 CFSv2 forecasts.In short, even as next week looks mild across much of North America, a pattern change toward colder weather with increasing opportunities for an expansion of North American snow cover looks increasingly likely afterward.

This from Bob Chill (ok, not his real name... he use to work for The Weather Channel)

Excellent GEFS run today. Very supportive of the cold outbreak. Growing support for a -nao down the line. Check out some of these members.

Looks like lots of storm threats 12/20 through early Jan. GEFS day 16 pattern, very stormy looking lots of potential. pic.twitter.com/BYjHfc3QYY

And lastly, a post from Eric Webb. Eric is a weather wiz if there ever was one and it's been great to latch on to his vast knowledge. He is very detailed in his work, and loves to explain weather to all of us weather geeks. :-)

Image from Eric Webb @webberweather

As I've shown w/ this graphic many times before it doesn't matter what the intensity or placement of the ongoing El Nino or warm ENSO Neutral Event is, January & February are more than likely going to end up colder than normal, more so than December, which is typically the most boring month in an El Nino winter anyway...Statistically, January & February are not only colder than December, but have more higher quality cold Januarys & Februarys, w February clearly the coldest of the 3... We've seen 36 El Nino events as defined by the ONI since 1900 (used 1971-2000 ONI climo for pre 1950 ONI record as noted by Bob Tisdale) (1899-1900, 1902-1903, 1904-1905, 1905-1906, 1911-1912, 1913-1914, 1918-1919, 1923-1924, 1925-1926, 1930-1931, 1939-1940, 1940-1941, 1941-42, 1951-1952, 1952-1953, 1953-1954, 1957-1958, 1958-1959, 1963-1964, 1965-1966, 1968-1969, 1969-1970, 1972-1973, 1976-1977, 1977-1978, 1982-1983, 1986-1987, 1987-1988, 1991-1992, 1994-1995, 1997-1998, 2002-2003, 2004-2005, 2006-2007, & 2009-2010)

Here are the statistics for Georgia, South Carolina, & North Carolina in the 108 winter months we've observed in all the El Ninos since 1900. I used the 1895-2000 base period for the pre-1950 El Ninos then used the 1981-2010 base period for the other events. I defined the months that were 1, 2, 3, & 4 standard deviations from normal as those if at least one or more climate regions in GA, SC, or NC had at anomalies that exceeded +/- 1 standard deviation, +/- 2 standard deviations, etc. February doesn't just look good compared to Dec & Jan visually, but also on paper. In fact, of all the El Nino Februarys that have been colder than normal, 16 of the last 17 (2003 as the only exception) have been at least one standard deviation or more below normal somewhere in NC, SC, & GA... (see chart above)Warmest El Ninos by month: December (1923), January (1952), February (1954) (See below)

I'll also mention that it's pretty rare to see December colder than normal in the southeastern US & then have both January & February end up warmer than normal & in all El Nino events since 1900, it's only happened 4 times (1952-53, 1953-54, 1997-98, & 2004-05) or about 10% of the time...I thought this would be applicable to the current discussion, I'm not saying we're going to see the exact same results, but I noticed several weeks ago after the anomalous upper level low brought what was virtually unprecedented snows to the SC midlands, the uncanny similarity to 1966, which if you extrapolated into the following phase of the Lezak Recurring Cycle, led to snow on the US east coast in time for Christmas... We're still a long ways out & a lot of uncertainty remains but combined w/ -VP associated w/ the active cell of CCKW once again entering the favorable 30-60E longitude & the model guidance of course which is finally starting to show some consistency, this is interesting nonetheless.

Summary

So as you can see, meteorologist are getting fired up about the upcoming pattern change and the weather to follow. I believe by the middle to the end of next week, we'll have a much better idea how this change is going to play out.

US Monthly DJF temps in accordance to placement/strength of an El Nino... Typical Warm Neutral-Weak Hybrid Dec on tap. Image from Eric Webb (Click to enlarge)

OK at least the end to the warm weather that's been driving everyone so crazy. This is December and we all have visions of a white Christmas and sleighs being pulled through the snow. :-) OK, it's me that has those visions. For myself, winter is the best season of the year, assuming you actually get some real winter weather to go play in. I grew up in Nashville TN and we had a little more snow there than here in Atlanta. I can still remember all of the great late 60's and early 70's winters and the large snowfall events. We would stay out for hours and hours at a time, only coming in to throw the wet clothes in the dryer and to get something warm to drink, then grab the sled and back outside for more fun. Ahhh... the good 'ol days.

So now people are getting antsy and starting to wonder if the much anticipated cold and snowy winter forecast that just about every meteorologist predicted is still coming? You haven't heard me talking much about the weather lately, but it's not because anything has changed, it's only because there really isn't much going on right now. Trust me... once we get cranking, you're going to get tired of hearing from me!

The pattern that we've been languishing in was actually expected and pretty much follows the textbook example of how a late fall early winter weak Modoki El Nino would behave (see the image to the left). But now we're seeing signs that this is all about to change. I'll let Larry Cosgrove explain from his most recent post.

After the barrage of seemingly endless computer model runs that indicated a long spell of mild temperatures across the continent, quite a few people who review or forecast weather threw out their winter forecasts. Out went the mostly "cold and snowy" descriptions, in came the terms of "blowtorch" and "balmy" for the DJF period 2014-2015. As I have often cautioned in releasing my predictions, outcomes shown by the numerical depictions are often short-term deviations from what eventually occurs.

So it should come as little surprise that beginning with theFriday12z December 5 model runs, signs of high-latitude ridging, southern branch storminess and a slow decline in temperature were seen in the 11 - 15 day time frame. These shifts continued throughSaturday, and affirms my belief that we are indeed headed into a "rough patch' covering the last week of December, and most of January, February into the first half of March. I have not changed my winter outlook one iota, for the reason that I thought there would be an extensive mild phase in this month that ultimately gave way to Arctic air and cases for more widespread snow.The reason why the current "El Nino" type configuration (split flow with mild ImP and mT air mass dominance) will shift to a highly amplified set-up featuring +PNA, -AO, and -NAO elements has to do with the active southern branch flow. What the models, and the analog formula, are saying is that a stronger disturbance will take shape in Texas, swing through the Deep South, and then move up along and off of the Eastern Seaboard. I am uncertain as to which particular storm will be the change mechanism, but the analog sample (20 members) points toward a new shortwave moving out of northern Mexico on December 17, translating to a colder, stronger low pressure center near Cape Hatteras NC on the 21st. Perhaps it is the same feature shown by the ECMWF model family over Texas in ten days. But I call your attention to the fact that when you compare ALL of the numerical versions to the analog-derived forecast, you see how very similar the maps are. Most notable: besides the storm threat, check out the extensive high-latitude ridge display. Yes, +PNA, -AO, -NAO coming up on tap!

I am not predicting a rapid change to a colder scenario in the U.S., but insist that the southern branch storm system will be the catalyst. This impulse may become an entrenched cA vortex over QC and ON at some point in the last week of December, after bringing copious amounts of rain with some inland snows. The snowpack should start to increase in coverage over the Midwest, Great Lakes and Appalachia during the 11 - 15 day time frame, leading of course to colder temperature forecasts east of the Rocky Mountains. I suspect we are going to be dealing with a mild West vs. cold Central and East set-up by Christmas Day.

All I want for Christmas is 498dcm cAk 500MB vortex over Lake Michigan....and a moderate Modoki El Nino with a persistent moisture fetch from the Caribbean Sea to the Grand Banks.

You can figure it out. :)Prepared by Meteorologist LARRY COSGROVE on Saturday, December 6, 2014 at 8:55 P.M. CT

Disclaimer: The previous statements are my opinions only, and should not be construed as definitive fact. Links provided on this newsletter are not affiliated with WEATHERAmerica and the publisher is not responsible for content posted or associated with those sites.

So as you can see, it appears we are just now entering the transition phase into winter that will last for about 2 weeks before we get into the heart of what still looks to be an awesome winter for us here in the southeast.

I also want to include a forum post by a gentleman named Eric Webb (Twitter @webberweather). Eric has an astounding amount of weather knowledge and freely shares and educates at the same time. While this may be over the head of some, it still shows the amount of detail and knowledge that goes into predicting weather. What he is talking about here are stratospheric warming events, and how they contribute (or not) to the actual winter weather on the ground. We have had a minor warming event already, but there are signs that a new SSW event may occur toward the end of the month. Sudden stratospheric warming tends to disrupt the Polar Vortex and, depending on the strength or the warming, can even change the flow of the stratospheric winds from east to west. This disrupting of the Polar Vortex cause high latitude blocking to form and can cause very cold air to spill into the lower 48 states. Not necessary for cold weather, but rather icing on the cake.

I should mention that since the satellite record only goes back to 1979, thus we're working with a very limited dataset and in general, a statistically significant and confident relationship between the variables may not be easily attained. Also keep in mind the stratosphere is not necessarily a cause of the pattern but merely a reflection of largely the background intraseasonal-interannual forcing (with some room for persistence on longer timescales), especially from the troposphere and you can throw some outside solar and QBO influence into the mix. However, as you'll see below, I don't think it's a huge loss whatsoever if we don't see a SSWE, (even though I'm definitely in favor of an event occurring within the next several weeks) after all, the robust +PDO/El Nino combination we currently have is one of the most favorable base states regardless of AO/NAO, etc. for the southeastern US...

Image from Eric Webb

Even if we don't see an event, we're still more than likely going to end up below normal temperature wise for the winter as a whole despite that December (as anticipated) was relatively mild/seasonable. In fact, looking at all the El Nino years with a +PDO in Sept-Oct (1902-03, 1903-04, 1911-12, 1913-14, 1918-19, 1923-24, 1925-26, 1940-41, 1941-42, 1957-58, 1958-59, 1965-66, 1969-70, 1972-73, 1976-77, 1982-83, 1986-87, 1987-88, 1991-92, 1997-98, 2002-03, 2004-05, 2009-10) only 3 out of the 23 events (1991-1992, 1997-1998, & 2004-2005) were above normal in the southeastern US in both January and February using the 1895-2000 climatological base period. That equates to about 85-90% of +PDO El Nino years having at least January or February colder than normal, ~70% of those years (1902, 1904, 1912, 1924, 1941, 1942, 1958, 1966, 1970, 1973, 1977, 1983, 1987, 1988, 2003, 2010) were colder than normal in both January and February in the southeast. I like those odds...And it gets better, if you look at the El Ninos with +AO/+NAO broken down by month (of course El Ninos are already biased towards the negative end of the NAO/AO indices, which is quite evident in seeing there are 5 or less El Ninos with a +AO in Feb & +NAO in Jan/Feb) you see that December & January as expected are warmer than normal, but amazingly, even with a +NAO &/or +AO, the February composite is still colder than normal, quite a testament to the strength of the ENSO forcing which often is fully capable, especially later in the winter, in overwhelming an unfavorable +AO/NAO signal...

Image from Eric Webb

All forcing mechanisms aren't necessarily created equally and you likely wouldn't know this tidbit about +AO/NAO's in an El Nino by simply looking at the US PSD temperature correlations for the NAO/AO, which both max out with a positive correlation of ~.5 to February temperatures in the southeast while NINO 3.4 & Tropical Pacific SST EOF only come to about .4 & .3 respectively. Superficially it seems temperatures should be above normal in such a pattern, but this is clearly not the case..In general it's fairly difficult to get the NAO to go positive in January or February and the AO in Feb, and and even if the NAO/AO go positive, it doesn't prelude wintry weather, as was the case in February 1973 and 2003, with the massive President's Day snowstorm in 2003 and the big one that crushed the southeast in 1973 during the 2nd week of February... The winter of 1972-73 featured a remarkable contrast in snowfall on the US eastern seaboard with many areas of the southeastern US that were the recipients of the big snowstorm in mid February seeing a lot more snow than the big cities of the northeastern US, with at least Washington DC (.1 inch), Baltimore (1.2 inches), & New York City (2.8 inches) experiencing their least snowiest winters on record...http://www.erh.noaa....bwi/bwisnow.txthttp://www.erh.noaa....dca/dcasnow.txthttp://www.erh.noaa....onsnowfall.html

Image from Eric Webb

Image from Eric Webb

Now, looking at the AO/NAO indices and how they stack up, I found little to no correlation in the winter as a whole (that doesn't mean that one isn't there, and we know from observing events like Jan 2010 & December 1984 that a SSWE has the capability to strongly modulate the Northern Annular Mode for at least a few weeks or so at a time, and I would likely best describe the forcing from stratospheric warming similar to solar i.e. elusive, although not nearly to that extent.) There's no doubt we need perhaps a few more decades of data at the very least before serious research can be undertaken. What surprised me a bit was that El Ninos without a stratospheric warming event actually had a lower overall seasonal AO. It's clear to me in looking over the data which I've shown below, it's not impossible to see a month or two with a huge -AO even though we don't see a big warming event, this is evident in January 1979, 1998, February 1988, and March 1980...

+ENSO Years AO w/o a SSWE (Image from Eric Webb)

The same can be said for warm ENSO years, there were quite a few positive months in this regime, in fact, take the winter of 2002-03 for example, we had a decent SSW event in January, which didn't do much, if anything to the AO...

Levi Cowan also made a very nice video a few years ago in the winter of 2011-12 during what even I would willingly quantify as a blowtorch about stratospheric warming events and ENSO. It's not difficult at all to get cold into the eastern US following a SSW event in an El Nino, La Ninas on the other hand as he mentions, with help from the cold PDO, often resist cold from a SSW event, but there are exceptions like 1984-85 & 1998-99...http://www.tropicalt...pheric-warming/On a side note, these Warm ENSO Neutral-Weak Hybrid El Nino years have been pretty much on target over the past 2 months, although just by viewing the 500mb for October and November, our AO looks like it will end up similar to or perhaps even lower than these years, which were in their own right quite a bit of fun...

Well... if you made it this far down the page, you should be much more weather savvy than when you started! You can also see we are just beginning to enter the fun times that await us this winter. You going to be hearing much more about this starting this week coming up, but certainly the following week (12/14-12/21).